1. Field of the Invention
The present invention relates to optical communication and more specifically, to an arrayed waveguide grating (AWG) wavelength division multiplexer.
2. Description of the Related Art
In an optical communication system, an arrayed waveguide grating serves as a filter for dividing or combining one or more optical wavelengths. Such an arrayed waveguide grating may also be employed as a bi-directional wavelength filter that is capable of bi-directionally diverging lights of different wavelength bands do not interfere with each other when they intersect one another. An AWG is conventionally formed by modifying the configuration of an existing concave diffraction grating of reflection type into that of a transmission type. A concave diffraction grating of reflection type follows a well-known Rowland circular configuration, which was proposed by H. A. Rowland in 1882.
FIG. 1 illustrates a typical Rowland circle configuration, wherein there is a concave diffraction grating 110 of reflection type and a Rowland circle 120 corresponding to the grating. In such a configuration, a position A of an incident point and a position B of a focal point of outputting lights (or an exiting point) may be varied depending on one or more design variables for configuring such a concave diffraction grating 110. However, they can not deviate from the Rowland circle 120 in any case.
By using such a relationship between a concave diffraction grating and a Rowland circle, it is possible to configure an AWG of transmission type. That is, an AWG of transmission type is configured in such a way that a concave diffraction grating of reflection type having an incident point and an exiting point on one Rowland circle is divided into two sections (input and output sides), thereby forming two plane waveguides (typically, each referred to as “slab waveguide”) of a Rowland circle configuration, and a plurality of channel waveguides are connected between the two plane wave guides.
FIG. 2 shows a configuration of a typical AWG. The AWG 200 comprises an input waveguide 210, a first slab waveguide 220, a plurality of channel waveguides 230 forming an arrayed waveguide region (AWR), a second slab waveguide 240, and a plurality of output waveguides 250. The input waveguide 210 is connected to a first end face 222 of the first slab waveguide 220 and the channel waveguides 230 connect a second end face 224 of the first slab waveguide 220 and a first end face 242 of the second slab waveguide 230. The plurality of output waveguides 250 are connected to a second end face 244 of the second slab waveguide 240 (the second end face being typically referred to as “image plane”). The first and second slab waveguides 220 and 240 separated into the left and right sides from the AWG 200 are formed by dividing those having theoretically been united in a single body. Therefore, the AWG 200 has a basic configuration (consisting of the first and second slab waveguides 220, 240 and the channel waveguides 230 ) which is exactly left-right symmetric with reference to centerline 260 of the channel waveguides 230 (or the centerline of the AWG). However, it is possible to optionally select the number and arrangement of the input waveguide 210 and output waveguides 250 as needed. In addition, because the physical basic configuration of the AWG 200 is symmetric, the operating characteristics of the AWG 200 also have reciprocity regardless of directions.
Because an AWG is typically used in one direction only, it has a same characteristic due to its reciprocity regardless of being used in any direction. Therefore, the difference between the input side and output side in an AWG are not results of a difference in configuration but determined according to how to use the AWG. Because such an AWG is normally used in one direction for one wavelength band, the conventional configuration may be appropriate without any problem in such a case. However, if it is intended to use such an AWG in both directions, i.e., if it is intended to use the AWG in such a manner that it operates upon different wavelength bands in different directions, a problem may be caused. Because such a conventional configuration has no difference in direction, it would seem natural that it can be made to operate for one of two wavelength bands.
FIG. 3 is a graph for illustrating output characteristics of the AWG shown in FIG. 2. FIG. 3 shows a first output spectrum 310 of light operating in O-band, of which the central wavelength is 1295 nm, and superimposed thereon a second output spectrum 320 of light operating in C-band, of which the central wavelength is 1550 nm. As can be seen from the FIG. 3, the O-band output spectrum 310 shows 30% decrease in bandwidth as compared to the C-band spectrum 320, although there is little difference in insertion loss between the O-band and C-band output spectrums. Such a result is caused by the fact that when an AWG designed for use in one wavelength band is used in another wavelength band, its physical properties, e.g., refractive index, are substantially changed due to the change of wavelength bands and, thus, its operating characteristics are also proportionally changed. Because the AWG 200 is a component having a symmetric basic configuration, it is difficult to find a method for counterbalancing such a phenomenon.
As described above, there is a problem in that a conventional AWG is not suitable for a bi-directional dual-band AWG which operates on different wavelength bands in different directions, respectively, because its basic configuration is left-right symmetric.